Control systems are in wide use and, as a general statement, function to "multiply" the relatively modest efforts of a machine operator (who manipulates a control device) to provide high-force or high-power output functions well beyond the physical capability of such operator. Such control systems are found in mechanical, electrical and fluid (hydraulic and pneumatic) forms and hybrids of those and other forms. Examples within common experience include the power brakes on an automobile and the electric switch capable of positioning an auto driver's seat (with the driver seated) with but a light touch on such switch.
Another example involves mobile machinery on which various "output" functions are controlled by the operator, often using some type of control system. A more specific example is a front end loader, a type of construction and earthmoving machine. A leading manufacturer of front end loaders and other machinery is Case Corporation of Racine, Wis.
Front end loaders, often mounted on rubber tires, are used to move dirt, rubble and almost any other type of material capable of being picked up by a shovel-like bucket mounted on the front of the machine. In northern climes during winter, such loaders are routinely seen removing piled snow from city streets and loading it into dump trucks.
Front end loaders (as well as many other types of mobile machines) have separate hydraulic systems including separate engine-powered hydraulic pumps which provide hydraulic oil under pressure for vehicle steering, for positioning implements, e.g., the vehicle bucket, and for operating the vehicle brakes. In most modern loaders, the implement-positioning system typically uses hydraulic cylinders for actual implement manipulation. Hydraulic oil under pressure is ported to and from the cylinders by directional valves.
In a larger machine, the implement system is aptly described as a "high horsepower" system in that significant fluid flow rates and fluid pressures are required. Therefore, the directional valves must themselves be large and able to "conduct" at high fluid flow rates and to withstand high flow and high pressure forces. Disregarding the problem of operator hand-to-valve "linkage," such valves do not lend themselves well to manipulation by hand.
In such machines, the directional valves may be positioned away from the operator's compartment (or at least out of easy operator reach) and are constructed to be positioned for bucket raising and lowering, for example, by a hydraulic control pressure rather than by a direct mechanical linkage or the like. Such "pilot operated" valves (as they are often called) have been in wide use for decades.
Such large, high-horsepower pilot operated directional valves are positioned by relatively small, low-effort controller valves, the handles or levers of which are placed to be within easy reach of the machine operator. Using a source of hydraulic pressure as a "power source," such controller valves direct fluid under pressure to the directional valves to position them to, e.g., raise or lower the bucket. In the absence of this power source (or "pilot pressure" as it is often called), the valve spool returns to its center or neutral position under the urging of springs within the valve. Sources of such hydraulic pilot pressure have included the steering and/or implement systems.
These earlier arrangements are characterized by certain disadvantages. In appreciating these disadvantages, it will be helpful to appreciate that fluid pressure (hydraulic or pneumatic) may be developed only if there is fluid flow and resistance to flow.
Consider the analogy of a common garden hose. If the hose has no nozzle and flow is therefore substantially unrestricted, the pressure at the open end of the hose is quite low and water flows freely. On the other hand, if a nozzle restricts flow, the pressure at such end rises appreciably as flow is progressively restricted and/or as flow rate progressively rises. And in either case, when the faucet is nearly or entirely shut off and little or no water flows, there is substantially no pressure at the open end.
It will also be helpful to appreciate aspects of common implement and steering systems. On a front end loader, the pressure in the implement system is a function of the rotational speed of the pump and of the flow "resistance" in the system. When the implement is idle (e.g., a loader bucket is not being raised or lowered), the directional valve is in "neutral." In the neutral position, fluid flows relatively freely through such valve and the system pressure is low, perhaps well below 500 p.s.i. Similarly, if the engine is at low idle and the pump is rotating relatively slowly, system pressure may be very low.
And many vehicle steering systems are not at significant pressure unless the system is being used. That is, system pressure may be very low unless vehicle turning is actually occurring.
Regarding the above-mentioned disadvantages, the operator-manipulated controller valves must have sufficient pilot pressure available to "shift" the large directional valves to, e.g., the bucket raise or bucket lower position. When such pilot pressure is obtained from the implement and/or steering systems, there can be times when such pressure is insufficient for directional valve shifting. This disadvantage can (and does) manifest itself in unusual and annoying ways.
One way a front end loader is commonly used is to smooth an earthen surface by, perhaps, spreading loose dirt across the surface or by smoothing already-loosened dirt. In so doing, the operator positions the loader so that loose dirt is behind the bucket. The bucket directional valve is then placed in the "float" position (using a pilot-pressure controller valve) and the bucket "back-dragged" (by driving the vehicle backwards) so that the curved rear portion of the bucket smooths the earth.
It is to be noted that retention of the directional valve in the "float" position requires that its flow-directing spool be retained at such position by a pilot pressure of some minimum value. Absent such pressure, the directional valve shifts (under the urging of its internal springs) to another position, the bucket is forced downward and the surface intended to be smoothed is, in fact, gouged. Such an eventuality can be disconcerting to the operator and takes valuable time to repair.
Described more broadly, the absence of a consistently-available pilot pressure source may prevent the vehicle from being used to its fullest, most productive advantage.